more heresy (my sb responses)

[Delwin D Fandrich]

Stephen Birkett wrote:
> 
> Frank and Del:
> > The wood the old guys had access to is no longer available now we have
> > abused our forest resources. We piano builders have to make do with
> > inferior wood.
> >
> Birkett:
> Couldn't be more untrue for pre-1850 pianos (also harpsichords). These
> guys didn't care about the wood like we do now. Rings per inch, perfect
> quartering, knot- and blemish-free wood....these ideas hadn't been
> invented back in 1814. If you look at old soundboards of this vintage
> there is wide variation in rings per inch, quartering can be as much as
> 15-20 deg off vertical, knots and pitch pockets are present. They used the
> same wood, either spruce or fir, that they used for the framing...just
> extracting the reasonably quartered boards. No fuss. No bother. Same wood
> we have available in my local lumber yard. More heresy. If you look back
> further to the harpsichord makers, they cared even less for wood "quality"
> in their soundboards. The most perfect instruments often had wild wood by our
> standards...pitch pockets and knots easily concealed under the soundboard
> painting. We have defined an arbitrary (visually based) standard for soundboard
> wood, then we complain because we can't get wood of that type. It may have
> been easier to get back in 1814, but it certainly wasn't used by piano
> builders. (It is not necessary for acoustic reasons.)

Actually, Frank’s and my statements are true. We were both speaking of
North American piano builders working from the late 1800’s through,
perhaps, the first third of the 1900’s. These builders did indeed have
access to wood of a quality that we can only dream about today. On the
North American continent, at least. I can’t speak for Europe. As to
whether earlier builders saw the need to use this wood or not is another
matter. Most of the popular wood mythology that pertains to our “modern”
understanding of the soundboard comes, I think, from the book “Piano
Tone Building,” a compilation of the “Proceedings of the Piano
Technicians Conference: Chicago 1916, 1917, 1918 and New York 1919. For
the full dose read the chapters on “Wood and the Piano Builder’s Art”
and “Sounding Boards.” These are basically transcriptions of lectures
given by Dr. E. W. D. Laufer, associate agriculture commissioner of the
American Steel & Wire Company. Most of the misinformation about
soundboards that we have today is articulated in these writings. Along
with some very good information as well, of course. 

I do agree with you in that I have also seen soundboards in instruments
built both prior to this era and during it that used somewhat less than
“soundboard quality tone” wood. I also agree that many of the
specifications that our early builders laid out for their “tone
wood”—the more I learn about soundboards, the less I like that
phrase—were arbitrary and unnecessary. It’s mostly a matter of
stiffness, weight and internal friction. I don’t think, however, that
all of what we now call “arbitrary” specifications were imposed for
strictly aesthetic reasons. They truly believed. Most of them, anyway. I
also have in my shop a soundboard that was made during the early 1900’s
that is made up of relatively short boards joined with kind of a tricky
little “half-lap” joint. The rest of the piano has been converted into
firewood, but not because of the soundboard panel, which still seems to
be quite nicely intact. I can’t speak to how well it performed its
function as a soundboard, since I never heard the piano play before it
warmed someone’s home in another fashion, but I can think of no reason
why it shouldn’t have worked reasonably well.

In this country, though, and especially through the era we were
discussing, great emphasis was put on selecting just the right wood for
the soundboard. And there was much to choose from. This is no longer the
case.


> Del:
> > Compression crowning is a process that is hard to control, leading to
> > a high failure rate.
> >
> Birkett:
> Couldn't agree more. It seems like a recipe for self destruction crowning
> a board with this technique. More heresy but why not try crowning boards
> the old way? These boards were dried below any RH they are likely to be
> exposed to, flat...then crowned by differential wood movement. Compression
> crowning a board at high RH seems to be asking for trouble...especially
> when RH goes low and the varnish seal gives up the goat. J. Bluethner
> crowned his boards the old way in 1885, and these were modern pianos (and
> they were varnished).  The Bluethner I had recently (c1883) still had
> adequate crown and had had no previous maintenance. (Caveat...you cannot
> dry a titebond glued board to 60C or the glue will fail, hide glue is fine
> at high temp.)

I think you may be misunderstanding my use of the phrase
“compression-crowning.” This is a phrase I coined some time back in my
writing and teaching about piano soundboards and how they are made and
how they work as acoustic transducers. There didn’t seem to be any term
in common use to describe the most common method of soundboard crowning
used from the late 1800’s through the mid-1900’s and I couldn’t think of
any other term to clearly describe the process

It refers to the practice of using artificially induced internal
compression to form and maintain crown in a piano soundboard. It is
accomplished by drying the soundboard panel to between 3.5% and 4.5%
moisture content prior to gluing on the ribs. The ribs may be either
straight or slightly curved (“crowned”). This moisture content is held
until the glue bond between the board and rib is fully set. Once the
soundboard is exposed to the ambient climate of the factory—and later,
the owners home—it absorbs moisture from the surrounding air and begins
to swell. Most of this swelling, as is the nature of wood, takes place
across the grain. Since the ribs now act to restrain the swelling of the
wood fiber and keep board from expanding as it normally would, it
creates a stress interface along the glue line and bellies the assembly
into a curve—”differential wood movement,” if you will. The assembly is
now crowned and the crown was formed, and must be maintained, by the
internal compression of the wood fiber within the soundboard panel.

This process was not invented just for the benefit of the piano builder.
It was the natural process used by all furniture makers assembling large
wood structures with animal hide glue. The wood had to be heated prior
to applying the hot glue and heating the wood dried it out. In fact, I
doubt that the practice of “crowning” soundboards was originally
designed into the piano. I think it developed as a natural outcome of
the ribbing process that was developed to efficiently use hot animal
hide glue.

As with all things, this practice has both good and bad consequences.
Obviously, the process works in that it produces a soundboard panel that
is capable of producing very good tone. At least most of the time. There
are some problems, though. The soundboard wood must be dried to a
moisture content several points lower than anything it will naturally
encounter in the varied North American climates the piano might be
exposed to. This means taking it down to something around 4.0%, ±0.5%.
Unfortunately this means that the wood will ultimately be subjected to
somewhat more internal compression than it will able to handle and
sooner or later the wood fiber will be permanently damaged by crushing.
Once this has happen the very property of the process that made the
panel work well as a soundboard— its resiliency; the springiness
component of the wave impedance equation—has been permanently lost.
Sometimes specific areas of this crushed wood fiber are visible in the
form of what we call compression ridges. Other times, the damage is not
readily visible, which is why the physical appearance of a soundboard
panel is not a good indication of it’s value as an acoustic transducer.
I hate to keep repeating myself here, but once again, this is fairly
basic wood technology. Not even piano builders are given the right to
suspend the basic laws of physics. Not even temporarily. 

Regards,

ddf